The ultimate goal of this project is to assess the significance of endocrine disruptors that increase thyroxine (T4) glucuronidation on thyroid carcinogenesis, because many endocrine disruptors are suspected to be thyroid tumor promoters. The mechanism by which endocrine disruptors promote thyroid tumors has been proposed to result from alterations in the hypothalamus-pituitary-thyroid axis. Endocrine disruptors alter the hypothalamus-pituitary-thyroid axis by increasing T4 glucuronidation and elimination, which reduces serum T4 as a compensatory feedback mechanism, thyroid stimulating hormone (TSH) will be released from the pituitary, which will stimulate the thyroid and result in thyroid cell proliferation and neoplasia. However, the preliminary studies suggest that a number of endocrine disruptors (3MC and PCB) interfere with the normal hypothalamus-pituitary-thyroid axis because these endocrine disruptors do not increase serum TSH. Therefore, the central hypothesis of this application is that endocrine disruptors that increase T4 glucuronidation are thyroid tumor promoters only when they increase serum TSH. To test this hypothesis, there are five specific aims: (1) This aim is to test the hypothesis that endocrine disruptors, which increase serum TSH, produce thyroid follicular cell proliferation via proliferating cell nuclear antigen (PCNA) immunocytochemistry. In aim (2), the hypothesis that the tumor promoting effects of endocrine disruptors are not correlated with the decrease in serum T4, but with the increase in serum TSH will be tested in a 25-week bioassay. Rats will be given the thyroid initiating agent, N-bis(2-hydroxypropyl)nitrosamine (DHPN), followed by exposure to endocrine disruptors. This study will provide critical information on the relationship between thyroid hormone imbalance, TSH secretion, and thyroid tumor promotion of rats treated with endocrine disruptors. Aim (3) is to test the hypothesis that endocrine disruptors decrease plasma T4 pharmacokinetically by increasing its glucuronidation. The pharmacokinetics of T4, as well as T3, will be determined to understand the mechanism(s) by which endocrine disruptors decrease serum T4 in the final aim (4), the mechanism by which 3MC and PCBs "blunt" the TSH response to reduced serum T4 will be examined, testing physiologic, pathologic or thyroid receptor binding mechanisms. If the overall hypothesis is true, then it has important implications in toxicology, for many endocrine disruptors have been shown to reduce serum T4. However, their effect on TSH is, at best, variable. The expectation is that increases in serum TSH, rather than reductions in serum T4 is a better indicator for thyroid tumorigenicity resulting from exposure to endocrine disruptors. If the investigators demonstrate that TSH mediates endocrine disruptor thyroid tumor promoting activity, then as long as the promotional mechanism is prevented (increase in serum TSH), cancer would be prevented.